Semi-knowledgeable roadie, but mtb noob

I'll just answer the q's in the sticky:

1) Your budget: <$500
2) What bikes, if any, are you already considering? Stumpjumpers seem to be recommended highly.
3) What type of riding do you intend to do? Cross country, I guess. The fire roads in nearby parks.
4) Do you have a preference over a hardtail or full suspension? It's my impressions a HT is the only way to go sub-$1500
5) Age, weight and height: 5'11" 200lbs
6) What sources will you consider buying from? Craigslist only, really. I don't need a new bike, and I have had enough eBay hassle to last me a lifetime.
7) Do you want people to offer you alternative suggestions to issues such as budget, bikes already considered, and sources? Dear lord yes

A little more:
I've been riding a ~10 year old, steel Novara Alight for a couple days now, as my roadie is incapacitated. Turns out I love riding around the fire roads in nearby parks (Tilden in Berkeley, for those from the bay area).

My above budget is a guess more than anything, I'm a student and very, very short on cash, especially for bikes. The only thing I can really say is that with road bikes, many people (myself included) feel 105/Rival is the sweet spot for price/benefit, and is a happy medium for "race worthiness" and durability.

I have no clue if this sort of standard exists in mtb. I know it's hard to recommend 1 bike if you're buying used, so knowing what group/frames/forks (anything else?) are in that range is probably the most useful. Any help/advice is appreciated.

I would probably try to stay with a 19 inch bike or maybe 18 inch if at all possible. If you can move your budget up a little bit you can probably find a few more bikes. For $500 you should be able to find a bike with 27 Speeds and Disc Brakes. REI had one for $469 recently.

Once you start looking stay focused on size. I also would try to stay with 2000 and newer and 27 speeds . If the deal was good enough I might go with rim brakes. Almost all the stuff from 2005 and newer will have disc brakes.

Here is an example of a Stumpjumper for $500 thats I think is a pretty good deal. You would have to convince someone to go on a road trip though since its east of Sacramento. If the frame is 19 inches it might be worth the trip because the price and size are really hard to find unless you are very patient.

105 would be closer to LX. For mountain bikes its XTR, XT LX, and Deore. For road bikes it Dura Ace, Ultegra, and 105. For Forks the top two are either a Fox or Rock Shox Sid, or Reba. If you want to step down a bit the Rockshox Recon is decent. You wont find a bike for $500 very often if at all with a decent Fox fork on it.

Heres another example of a full suspension Stumpjumper that a pretty good deal that might work for you if you are the right height.